Long exposure to noise can damage the eardrum of the inner ear, causing permanent hearing loss. Even only exposing to a loud noise for a short period of time might cause discomfort. Recent reports show that today's young people seem to be experiencing hearing loss at an astonishing rate, and that personal audio equipment seems to be contributing to that trend as like any other sound, music can cause hearing loss if it is loud enough and exposure is long enough. Especially when earbuds or earpieces are often used with such personal audio equipment for audio entertainment, and users of such personal audio equipment are consciously exposing themselves to loud volumes while situating in a noisy environment, which poses a threat to noise-induced hearing loss.
There are two types of noise reduction earphones, which are generally categorized in terms of how they are worn by the user. These two types are referred to as around-the-ear earpieces and in-the-ear earpieces. Usually, a conventional around-the-ear earpiece is a bulky device that uses sponges as its acoustic damping materials, and resembles an earmuff that covers and surrounds ears of a user for passive noise attenuation. However, it is not easy to carry because of its large size. With regard to those conventional in-the-ear earpieces, they are designed to fit into ear canal so that they can be fixedly stuffed inside the ears for blocking out external noises. Unlike the around-the-ear earpieces, the in-the-ear earpieces are easy to carry as they are light and compact. However, although the in-the-ear earpiece can provide better acoustic isolation effect, it may cause ear discomfort since it can seal the ear canal completely and therefore cause imbalance in air pressure. In addition, it is sensitive to the so-called internal noises. That is, when a user having a conventional in-the-ear earpiece fitted inside his/her ears, the sounds of speaking, swallowing, muscle/joints movements, etc., are seemingly to be amplified and thus clearly audible to the user.
An improved headphone with active circuit design for noise filtering was provided in U.S. Pat. No. 4,455,675, entitled “Headphoning”. In U.S. Pat. No. 4,455,675, an acoustic control system is provided, which uses acoustic waves generated by acoustical sensing means for compensating and thus eliminating unwanted acoustic waves. The abovementioned technique had been vastly applied in related industries. Nevertheless, it can only be used for canceling out noises of low-frequency range, such as those of several kHz, but cannot be used for canceling out noises of high-frequency range since it cannot synchronize with the phases of those high-frequency noises. Therefore, the earmuff-like structure is still required for blocking out the high-frequency noises. There are many other noise reduction devices being successively disclosed thereafter, such as the one disclosed in U.S. Pat. No. 4,985,925, entitled “Active Noise Reduction System”, which may use electronic parts or circuit layouts different from those shown in U.S. Pat. No. 4,455,675. However, the primary design of using an active circuit for noise attenuation remains the same and thus they all fail in high-frequency noise cancellation.
FIG. 1 shows an in-the-ear headphone disclosed in U.S. Pat. No. 6,683,965, entitled “In-the-ear Noise Reduction Headphones”. The aforesaid in-the-ear headphone includes a shell 14 that has an extended portion 16 being shaped and sized to fit into the concha of a user's ear. In addition, an internal cavity 28 is defined in the shell 14 that is channeled with a passageway 29 extending through the extended portion 16. A speaker 32 is arranged inside the passageway 29 while arranging a microphone 34 in the passageway 29 at a position beneath the speaker 32. By the arrangement of the speaker 32, the microphone 34, and the acoustic connection between the passageway 29 and the ear canal, noise reduction can be achieved. However, as the cavity 28 and the passageway 29 are not structured to equip with filtering ability, noise of high-frequency range cannot be filtered thereby.
FIG. 2 shows a feedback type active noise control earphone, disclosed in TW Pat. No. 91213715. The feedback type active noise control earphone is primarily structured as a housing 140 having at least a speaker 110 arranged thereon, in which at least a microphone sensors 120 is installed around each speaker 110 for sensing ambient noise and thus converting the sensed noise into a noise signal to be received by the active noise control circuit 130 for enabling the same to generate a noise reduction signal. Therefore, each speaker 110 is enabled to produce an inverse phase audio signal with respect to the noise reduction signal. As each microphone sensor 120 is positioned in front of its corresponding speaker 110 while being arranged inside an energy vortex 150 generated inside the housing 140 by near-field effect, low frequency noise not only can be blocked from being received, but also can be cancelled by the inverse phase audio signal of the speaker 110. Nevertheless, the aforesaid earphone can only attenuate low frequency noise. In addition, not only the positioning of the microphone sensor is restricted to be placed in front of the speaker, but also the cooperation of the housing 140 and the active noise control circuit 130 is required.
FIG. 3 shows a headphone apparatus 10 with feedback type noise cancellation facility disclosed in U.S. Pat. No. 5,668,883, entitled “Headphone Apparatus Including An Equalizer System Having An Open Loop Characteristic With A Rising Slope Outside The Cancellation Band”. The headphone apparatus 10 of FIG. 3 includes an acoustic pipe 6, a loudspeaker unit 5, a microphone unit 9 and a feedback circuit. The acoustic pipe 6 has an inner diameter W substantially equal to that W0 of an external auditory canal A. The acoustic pipe 6 has a mounting portion provided at an end thereof for being mounted on the outer ear and has an acoustically non-reflective end at the other end thereof. With the aforesaid headphone apparatus, since the open loop characteristic of the equalizing section 3 by way of which the output signal of the microphone unit 9 provided on the acoustic pipe 6 having an inner diameter W substantially equal to that W0 of the external auditory canal A is fed back to the loudspeaker unit 5 also provided on the acoustic pipe 6 is set to the characteristic, wherein the attenuation characteristic outside the frequency band in which noise can be canceled rises higher than the attenuation characteristic in the frequency characteristic in which noise can be canceled, the noise attenuation amount can be increased and the frequency band in which noise can be canceled can be widened. In other words, the headphone apparatus 10 of FIG. 3 includes amplifier set-up so that the gain characteristic outside cancellation band corresponds to the open loop characteristic. The output of the microphone unit 9 is fed back to the signal input system to the loudspeaker unit 5, thereby constructing a noise cancellation circuit of the feedback type. Noise Pin admitted into the inside of the acoustic pipe 6 of the headphone apparatus 10 from the outside is examined here. A sound pressure Po acting upon the ear-drum B is given, from the character of feedback, and the noise Pin arrives at the ear-drum B after it is attenuated by an amount corresponding to the loop gain.
FIG. 4 shows an earplug for selective filtering of sound transmission into the external auditory canal, disclosed in U.S. Pat. No. 5,832,094, entitled “Device Of Transmission Of Sound With Selective Filtering For Insertion In The Outer Auditory Canal”. The sound transmission device with selective filtration for being placed in the external auditory canal of a user, as shown in of FIG. 4, includes a plug 1 provided with a hole 2 and an acoustic valve 8 at least partially within said plug 1. Said plug 1 is fittable in the auditory canal of the user. The device comprises a tube 3 which opens at its inner end into the residual cavity 7 existing between the plug 1 and the eardrum 4, and opening at its outer end into the acoustic valve 8. The acoustic valve 8 defines at one resonance cavity 10, 11, wherein said residual cavity 7 and said acoustic valve 8 are acoustically coupled by said tube 3 so as to form a fourth-order acoustic filter. Furthermore, the tube 3 extends through the plug 1 and opens into a space defined by the plug 1 and the eardrum 4 of a user. The opposite end of the tube 3 is connected to an acoustic valve 8 which is partially or wholly inserted in the plug 1 and contains one or more resonance cavities 10, 11. This invention is to provide a sound transmission device with selective filtering in the form of a plug that completely blocks the outer auditory canal. The plug includes an acoustic valve and an open tube associated with at least one resonance cavity of the valve. According to the well known HELMHOLTZ resonator principle, the acoustic filter thus obtained is a fourth order filter with an attenuation slope of 30 decibels per octave.